Are all of the regular Sunnah prayers obligatory to maintain, and must they be made up if forgotten, and is it better for one who sometimes prays them and sometimes leaves them off to abandon them entirely?

A Muslim is only obligated to perform the five daily prayers during the day and night, as stated in the Hadith of Talhah ibn Ubaydullah. A Muslim's performing of the regular supererogatory prayers sometimes and leaving them sometimes is better than abandoning them entirely. Preserving the Sunnah (Prophetic tradition) is better than abandoning it, for the Prophet, peace and blessings be upon him, said: "The most beloved of deeds to Allah are those that are most consistent, even if they are few."